Organic Carbon Storage Capacity by Association With Minerals in Tropical Alkaline Soils

Soil organic carbon (SOC) is essential for maintaining soil functioning as well as for mitigating climate change. SOC stabilisation in mineral‐associated organic matter (MAOM) fraction plays a key role, and MAOM distribution should be quantified in divergent environments to improve predictions of SOC dynamics. However, quantitative information on the OC storage capacity of MAOM based on MAOM distribution is still limited, especially in tropical alkaline soils, which might have a lower OC storage capacity because of weaker interactions between C and clay via cation bridges such as Ca and Mg, compared to common Al‐, Fe‐organo‐metal complexes in tropical acidic soils. This study aimed to evaluate the OC storage capacity of MAOM based on the relationship between the mass proportion (%) and OC content in MAOM (g C kg −1 soil) in 46 tropical alkaline surface soils of southern India with variable soil types (Alfisols, Inceptisols, and Vertisols) and land uses (paddy fields, croplands, and bushes). Boundary line analysis was used to estimate the upper distribution of MAOM‐C for our dataset, and the slope of the regression line (0.16, R 1 = 0.16, p < 0.001) was substantially lower compared to that in a previous review of tropic soils (0.53, R 1 = 0.46, p < 0.005) mainly conducted on acidic to neutral soils (Fujisaki et al., 2018). We found that MAOM‐C content was positively correlated with Ca ex , Mg ex , and clay, and was well predicted by Mg ex both in a simple and multiple linear regression model. Considering the alkaline soil pH of dataset (8.0 ± 0.1), these results indicate that a lower OC storage capacity in this study might be attributed to the weaker interactions between OC and minerals via cation bridges in tropical alkaline soils. There were no significant differences in the slope of the linear regression lines between soil types and land uses, except that the slope in Alfisols was greater than that in Vertisols, and all the slopes were consistently lower than that of Fujisaki et al. (2018), similar to that of the boundary line analysis for the whole dataset. Our results provide direct evidence that tropical alkaline soils have a substantially lower organic carbon storage capacity as MAOM than that of tropical acidic to neutral soils. Further study is necessary to elucidate the factors controlling the low OC storage capacity in this area.